It has been conventionally known that when oxide or hydroxide of cerium or zirconium is compounded into a silicone rubber, the silicone rubber to be obtained improves in heat resistance and compression set (see, for example, Patent Reference 1 (JP-B 1961-006189 (KOKOKU)) and Patent Reference 2 (JP-B 1962-000837 (KOKOKU))). Further, in order to impart better heat resistance and compression set to a silicone rubber, for example, a silicone rubber in which silica carrying oxide particle of cerium, nickel, iron, or cobalt is highly dispersed (see Patent Reference 3 (JP-B 1991-012586 (KOKOKU))) and a silicone elastomer containing cerium oxide having an increased BET specific surface area (see Patent Reference 4 (JP-A 1990-045564 (KOKAI))) have been suggested.
However, according to Patent Reference 5 (WO 2008/082001 A1), an uncured silicone rubber composition in which cerium oxide powder is compounded has a problem of increasing in viscosity or generating gel if it is let stand at a temperature above 70° C. Hence, Patent Reference 5 discloses that a heat curing silicone rubber composition excellent in thermal stability during manufacture and in an uncured state and excellent in heat resistance after curing can be provided by specifying the total light transmittance when a predetermined amount of cerium oxide powder is dispersed in an uncured silicone rubber composition.
Note that though preferable properties such as the surface characteristics are specified about oxide and hydroxide of cerium and zirconium in view of improvement in heat resistance and compression set of a silicone rubber containing the above in the conventional technique, the relation between the chemical structural characteristics of these compounds and the heat resistance and compression set of the silicone rubber containing them has not been made clear yet.
In such a situation, it has been desired to develop a silicone rubber composition capable of manufacturing a silicon rubber whose compression set in a higher temperature range, for example, in a temperature range of 200° C. or higher, is sufficiently suppressed by selectively using a chemical compound which is capable of improving the compression set in the higher temperature range by being compounded into the silicone rubber from among cerium compounds and zirconium compounds.